The present invention relates to a porous ceramic sheet, particularly to a porous ceramic sheet containing nickel and stabilized zirconia which is mainly used for a solid oxide fuel cell, to a process for producing the porous ceramic sheet, and to a setter for use in the process.
A fuel cell using stabilized zirconia as a solid electrolyte usually has a high working temperature of 900xc2x0 C. to 1000xc2x0 C., and is to be exposed to such high temperatures for a long time. It must therefore be composed of a material having an extremely high thermostability, and its anode and cathode must be composed of materials having a coefficient of thermal expansion almost identical to that of the material for the solid electrolyte. These materials can only be selected from a very narrow and limited range and from expensive materials. Accordingly, development has been made to provide solid oxide fuel cells containing a thin film solid oxide electrolyte and having a working temperature ranging from 600xc2x0 C. to 800xc2x0 C.
To develop fuel cells using a thin film solid oxide electrolyte, porous ceramic sheet having the following characteristics should essentially be developed. The required characteristics are, for example, high gas permeability, high electrical conductivity, a coefficient of thermal expansion close to that of the electrolyte, high mechanical strength, and satisfactory shock resistance at high temperatures (thermal shock resistance).
Therefore, a porous ceramic sheet composed of nickel oxide-stabilized zirconia is generally used. To improve the gas permeability of such a porous ceramic sheet, it is effective to lower the sintering temperature of the NiO-stabilized zirconia, and to enhance its electrical conductivity, it is effective to increase the Ni content in the NiO-stabilized zirconia. Contrary to this, to improve the mechanical strength of the ceramic sheet, it is desirable to increase the sintering temperature of the NiO-stabilized zirconia and thereby to decrease the porosity, and to make the coefficient of thermal expansion close to that of the electrolyte, it is effective to decrease the Ni content in the NiO-stabilized zirconia. As thus described, the porosity and electrical conductivity are somewhat inconsistent with the mechanical strength, and a porous ceramic sheet which is satisfactory in all these characteristics has not yet been developed in practice.
For instance, Japanese Unexamined Patent Publication No. 4-147569 discloses a ceramic sheet obtained by granulating a powdery mixture of nickel oxide and zirconia to give a granulated powder, forming the granulated powder, and firing the formed powder under a reducing atmosphere to give a ceramic sheet. In general, however, firing under a reducing atmosphere, particularly sintering at 1200xc2x0 C. or higher causes volumetric shrinkage during reduction, and the resultant product is liable to be cracked. Separately, Japanese Unexamined Patent Publication No. 5-225986 discloses a ceramic sheet obtained by granulating and heat-treating a powdery mixture of nickel oxide and zirconia to give a granulated powder, adding a given amount of a sintering aid to the granulated powder, forming the resultant mixture and firing at 1300xc2x0 C. to 1600xc2x0 C. under an oxidizing atmosphere. The addition of such sintering aids is effective for the improvement of mechanical strength. However, nickel oxide or zirconia is liable to undergo a solid state reaction with the sintering aid component at high temperatures to give a complex oxide of nickel oxide or zirconia with the sintering aid component. This complex oxide may deteriorate the gas permeability and electrical conductivity of the ceramic sheet with time.
Japanese Unexamined Patent Publication No. 8-287921 discloses the optimization of particle diameters of nickel oxide and stabilized zirconia to be used as material powders. To be more specific, it discloses a ceramic sheet using a nickel oxide powder having a particle diameter one tenth or less that of stabilized zirconia particles. This is intended to ensure the electrical conductivity and to meet the overall characteristics such as mechanical strength and coefficient of thermal expansion, by rendering nickel oxide particles come interstitially into gaps between stabilized zirconia particles to form an area where the proportion of Ni is high even when the content of nickel oxide is reduced. Such insurance of the electrical conductivity by uneven distribution of nickel, however, deteriorates the homogeneity in electrical conductivity of the porous ceramic sheet, and reduces with time the mechanical strength and thermal shock resistance in long-term use.
Separately, Japanese Unexamined Patent Publication No. 9-223508 discloses an NiO-stabilized zirconia porous ceramic sheet having pores about 1 xcexcm or less in diameter obtained by a tape casting technique.
The firing for the production of the porous ceramic sheet disclosed in the publication is performed at a high temperature of about 1400xc2x0 C., and therefore the nickel component is sintered to occlude or excessively narrow the pores or the proportion of nickel oxide is reduced by volatilization, which may deteriorate the gas permeability and electrical conductivity. The uneven distribution of the nickel component may affect changes with time of the mechanical strength and thermal shock resistance and may not meet the stability with time of the electrical conductivity.
When a ceramic sheet is produced by firing a precursor green sheet, it is generally produced by a process of placing a setter of, for example, alumina in a furnace, which alumina setter is available at low costs, and placing the green sheet on the setter. However, these porous ceramic sheets composed of nickel oxide-stabilized zirconia produced by such a process are liable to have decreased strength and electrical conductivity and hence have not meet the characteristics required for solid oxide fuel cell applications.
The invention has been accomplished under these circumstances, and it is an object of the invention to provide a porous ceramic sheet advantageously used in solid oxide fuel cells, which has a satisfactory thermal shock resistance and mechanical strength and exhibits a sufficient permeability against a fuel gas and a desired electrical conductivity, a production process for the ceramic sheet, and a setter for producing a porous ceramic sheet which can advantageously be used in the production process.
After intensive investigations of causes of deterioration in strength and electrical conductivity in a porous ceramic sheet, the present inventors found that when a green sheet, which is a precursor of a porous ceramic sheet containing stabilized zirconia and nickel oxide, is heat-treated on, for example, a conventional alumina setter, the nickel component in a surface layer of the green sheet scatters or migrates into the alumina setter, and decreases, resulting in reduced mechanical strength and electrical conductivity. The present invention has been accomplished based upon the above finding by improving a process for producing, in particular a setter for producing, a porous ceramic sheet.
To be more specific, the inventive setter for producing a porous ceramic sheet is a setter to be used for the production of a porous ceramic sheet containing nickel oxide and stabilized zirconia, which setter comprises a sheet ceramic containing 40% to 90% by weight of an [NiO] unit. The production setter may preferably be obtained by forming a slurry into a sheet to give a green sheet, the slurry containing 40% to 90% by weight of a nickel oxide powder as material powder, and subjecting the green sheet to a heat treatment at 1300xc2x0 C. to 1500xc2x0 C. In this connection, the [NiO] unit means nickel oxide itself or an NiO unit when nickel forms a complex oxide with another component.
The inventive process for producing a porous ceramic sheet includes the steps of: forming a slurry into a sheet to give a green sheet, the slurry containing 20% to 50% by weight of a stabilized zirconia powder and 50% to 80% by weight of a nickel oxide powder, placing the green sheet on the inventive setter, and firing the green sheet on the setter at 1200xc2x0 C. to 1400xc2x0 C. It is preferable that the top of the green sheet placed on the setter is covered with another piece of the inventive setter and the covered green sheet is fired. The inventive porous ceramic sheet is a porous ceramic sheet containing nickel oxide and stabilized zirconia, in which a ratio X of a ratio Xa relative to a ratio Xb ranges from 0.85 to 1.18, the ratio Xa is a ratio of an X-ray diffraction peak intensity of the (200) line of nickel oxide relative to an X-ray diffraction peak intensity of the (111) line of the stabilized zirconia on one side of the sheet, and the ratio Xb is a ratio of an X-ray diffraction peak intensity of the (200) line of nickel oxide relative to an X-ray diffraction peak intensity of the (111) line of the stabilized zirconia on the other side. To be more specific, it is a porous ceramic sheet produced by the inventive production process. The inventive porous ceramic sheet preferably has an area of equal to or more than 100 cm2 and a thickness ranging from 50 to 1000 xcexcm.